Process for purifying aqueous buffer solutions

ABSTRACT

The present invention describes a procedure for the removal of sodium sulphate from a phosphate solution that has been used for absorption of SO 2  from flue gases, whereby a precipitate forms when the absorption solution is regenerated by evaporation. In this procedure a part of the precipitate is separated from its solution and treated with water and SO 2  for transformation of disodium-hydrogen phosphate into monosodium dihydrogen phosphate in the aqueous solution, whereby the sodium sulphate remains undissolved and is removed by filtration, whereafter the filtrate is added back to the main process.

FIELD OF THE INVENTION

[0001] This invention relates to a process for purifying aqueous buffersolutions used for absorption of sulphur dioxide (SO₂) from exhaustgases by means of a regenerable SO₂ absorption process. The term“purifying” here includes the recovery of valuable buffer compounds froma buffer stream drained from the process solution, in order to avoidthat absorbed impurities reach unacceptable high concentrations. Themost important impurity in the present case is sodium so sulphate(Na₂SO₄), but other unwanted compounds may also be present.

BACKGROUND OF THE INVENTION

[0002] There is a strong impetus for developing regenerative absorptionprocesses for SO₂ recovery from gas flows. The tern “regenerative” inthis case usually implies that the active absorption solution isregenerated, and thus made ready for reuse. At the same time, theabsorbed SO₂ is set free in a concentrated, useful form. Suchregenerative processes are especially attractive for treating large gasflows with high SO₂ concentrations, when the otherwise commonly usedlime/limestone based processes are less suitable. SO₂ recovery is alsoespecially attractive when there is a local demand for SO₂, for examplefor sulphuric acid production, or when there is available a Claus plantfor producing elemental sulphur from H₂S, such as in oil refineries.

[0003] Important problems with regenerative SO₂-processes are:

[0004] Insufficient chemical stability of the active absorbent, leadingto too high reagent consumption.

[0005] Insufficient precleaning of the feed gas, which again leads tolosses of reagent solution.

[0006] The formation of unwanted sulphur compounds by unwanted sidereactions, especially sulphate formation by oxidation caused by oxygenbeing absorbed, from the feed gas.

[0007] Under certain conditions, dis-proportioning reactions may occur,producing, for example, thiosulphate.

[0008] The unwanted products accumulate in the buffer solution, and haveto be removed, in order to avoid operation disturbances caused byprecipitation through super-saturation. SO₂ oxidation first leads tosulphuric acid, which must be neutralized by addition of a base, forexample NaOH, in order to counteract acidification of the buffer. Thensodium sulphate forms.

[0009] One possibility of avoiding super-saturation of sodium sulphateis to drain a sufficient stream of buffer slurry from the evaporator.However, this may lead to a substantial consumption of valuablechemicals, and may—in addition—represent a disposal problem.

SUMMARY OF THE INVENTION

[0010] The goal of the present invention is to develop a new and simplemethod for purifying the buffer solution, without the drawbacksmentioned above. The present invention combines the use of a chemicallycompletely stable buffer system for SO₂ absorption, with a method forselective removal of sodium sulphate.

[0011] The aqueous buffer system utilized in the absorption processconsists of concentrated sodium phosphates The use of sodium phosphatesfor selective SO₂ absorption has been known from earlier patentliterature. Furthermore, it is known that normally very little sulphateformation through oxidation occurs in such solutions. Nevertheless, somesodium sulphate is expected to appear, caused by the presence of someSO₃ in the feed gas. This SO₃ has to be neutralized by means of aNa-base.

[0012] An obvious procedure for keeping the sulphate concentration lowenough is to withdraw a little stream of buffer solution, and substituteit with fresh buffer. For a 500 MW_(e) power plant with 0.25 vol.-% SO₂in the flue gas, the sulphate production will be about 2.0 kmol/h whenthe sulphate formation rate corresponding to 2% of the SO₂ absorbed. Ifthe sulphate concentration is then to be controlled only by withdrawalof process buffer, 4 m³/h must be removed in order to keep the sulphateat 0.5 kmol/m³. With typically 3 kmol/m³ of Na₂HPO₄ in the buffer, witha price set at USD 1.74 per kilo, alone the loss of Na₂HPO₄ will amountto USD 3000 per hour. Such draining of buffer thus leads to a highoperating cost. Added to this comes the expected environmental problemassociated with disposal of the resulting sodium sulphate-sodiumphosphate mixture. This example shows that there is a need for a methodfor selective removal of Na₂SO₄, without substantial loss of valuablebuffer.

[0013] Up till now, separating sodium sulphate from the process solutionwith sufficient selectivity towards sodium phosphate solution and withacceptable expenditures, has met with significant difficulties, and sohas the disposal of the solids removed.

[0014] The following reactions are utilized in the known phosphate-basedSO₂ process:

SO_(2(g))=SO_(2(l))

SO_(2(l))+2H₂O=H₃O⁺+HSO₃ ⁻

H₃O⁺+HPO₄ ²⁻=H₂O+H₂PO₄ ⁻

[0015] The mechanism is that the relatively basicmono-hydrogen-phosphate ions (HPO₄ ²⁻) react with the acid hydroniumions (H₃O⁺), which are set free when SO₂ is transformed into bisulphite(HSO₃ ⁻). Thereby, the more acid di-hydrogen-phosphate-ions (H₂PO₄ ⁻)are formed. The cation utilized is Na⁺, which is added in the form of asodium base, for example NaOH.

[0016] U.S. Pat. No. 4,948,572 describes a phosphate process where theregeneration of the buffer solution is by evaporation, whereby thereactions above are reversed, and gaseous SO₂ is driven off, togetherwith steam. The steam is then removed by condensation, leavingpractically pure, gaseous SO₂ as product, ready for production of, forexample, liquefied SO₂, elemental sulphur, or sulphuric acid. SolidNa₂HPO₄ mixed with Na₂SO₄ and sodium pyrosulphite (Na₂S₂O₅),precipitates in the evaporator. (Na₂S₂O₅ is the anhydride of sodiumhydrogen sulphite (NaHSO₃)). Regenerated buffer is made ready for a newround of absorption by addition of condensate and/or water to thesuspension, which is drawn off from the evaporator.

[0017] A simple procedure for limiting the sulphate concentration of thebuffer solution is to remove part of the precipitate, which forms in theevaporator. However, since this material will often consist of about 60%of sodium phosphate, a substantial loss of valuable material results,and disposal is problematic, especially for large plants with largeamounts of solids.

[0018] Selective removal of Na₂SO₄ with acceptable expenditures, hasthus met with considerable difficulties. One example, which maydemonstrate this, is described in NO patent 164218. The process isstarted with a side-stream S of the SO₂-rich buffer coming from theabsorption tower and being taken out for treatment. In the first step ofthe treatment, an adjusted, large part of the water content of thisstream is removed by evaporation. Thereby a precipitate, which isrelatively rich in Na₂SO₄, forms. Different extents of the evaporationwere tested, in order to find the most promising composition of theprecipitate. In a further detailed example, 1411 gram of water wasevaporated, together with 79.7 gram of SO₂. The evaporation wasperformed at atmospheric pressure, and the amount of water evaporatedrepresented 62% of the water content at the start. The precipitate wasseparated from the suspension by filtration, and contained 46% Na₂O₄.26% Na₂HPO₄, and 14% Na₂S₂O₅, beside some water. 65% of the startcontent of the Na₂SO₄ in the buffer feed to the evaporator was found inthe filter cake. A relatively moderate selectivity thus resulted,leading to a correspondingly high consumption of chemicals. In addition,the procedure followed involves a separate evaporation operation, whichsignificantly adds to the cost.

[0019] There is now found a new and unexpected simple procedure wherebya substantial more selective removal of sodium sulphate from thephosphate buffer is accomplished. The new method does not require aseparate evaporation step. Also, an unexpectedly high part of thesulphate present at start, is present in the final filter cake, which isseparated from the treated SO₂ process slurry, ready for disposal.

[0020] The present invention represents a procedure for removing sodiumsulphate from a sodium phosphate solution, which has been used forabsorption of sulphur dioxide from gas flows. A part of the precipitate,which forms when the loaded solution is regenerated by evaporation, isseparated from its mother liquor and thereafter treated with water andgaseous sulphur dioxide for transformation ofdi-sodium-monohydrogen-phosphate into mono-sodium-di-hydrogen-phosphate.In this operation, the dominating part of the sodium sulphate remainsundissolved, and is then separated from the aqueous phase by, forinstance filtration, and the filtrate is returned to the main process.

[0021] Following a preferred procedure, the process is carried out at anelevated temperature, preferably below 100° C., and most preferablywithin the range 40 to 80° C.

[0022] The procedure according to the invention requires that part ofthe precipitate that forms in the evaporation step of the main processin accordance with the U.S. patent mentioned, is separated from itsmother liquor, and then treated with water and SO₂ at an elevatedtemperature. The time required for this treatment, is reduced byincreased temperature. However, a temperature considerably below 100° C.is preferred, since the SO₂-treatment can then be carried out in asimple equipment and at atmospheric pressure. The preferred temperatureis in the range 40-80° C. By this treatment, Na₂HPO₄ is transformed intoNaH₂PO₄, which goes into solution, following the gross reaction,

SO_(2(gl))+H₂O_((l))+Na₂HPO_(4,(s))=NaH₂PO_(4,(l))+2 NaHSO_(3,(l))

[0023] By proper adjustment of the amounts of water and SO₂ added,Na₂SO₄ may remain almost quantitatively as solid. Thus, it can beselectively separated from the mother liquor and removed from theprocess, for example, by filtration or by centrifuging. At the sametime, the mother liquor, with its content of phosphate and bisulphite,can be returned to the main process.

[0024] It has been found that a small amount of sodium pyrosulphite,Na₂S₂O₅, will follow the precipitate, which mainly consists of Na₂SO₄.The main part of the pyrosulphite, along with mother liquor remaining inthe separated solid, can be retrieved by simple water-wash, using aproper amount of water. The temperature of the washing water ispreferably kept at the same level as the temperature of the precedingprocess steps. The washing process should follow the conventionalprocedure, whereby the solid remaining after washing, is separated byfiltration or centrifuging. The filtered washing water is returned tothe main process along with the mother liquor.

[0025] The importance of the invention is illustrated by the followingexample.

[0026] The phosphate process is used for recovering 1.5 kmol/h of SO₂ byabsorption from incinerated tail gas in a sulphur recovery plant (Clausplant), 1.5 m³/h of SO₂-rich phosphate solution goes to the evaporator,where SO₂ is driven off together with steam. Along with this, aprecipitate forms, in the amount of 185 kg/m³ SO₂-rich feed solution.Analysis of the separated solid after drying, showed 24 weight-% ofNa₂SO₄. The corresponding total amount of precipitated Na₂SO₄ is1.5*185*0.24=66.6 kg/h, which is equivalent with 66.6/142=0.469 kmol/h.

[0027] It must be added that the sulphate concentration of the phosphatebuffer of the main process, was kept at about 0.5 kmol/m³, bycontinuously withdrawing solids from the evaporated slurry. 2% oxidationof the amount of SO₂ recovered gives a sulphate generation rate of1.5*0.02=0.03 kmol/h. Keeping the sulphate concentration at 0.5 kmol/m³,then requires only (0.030/0.469)*100=6.4% of the precipitate of the mainprocess to be treated. However, with about 60 weight-% of sodiumphosphate in this solid, it is realized that a process separating thephosphate so that it can be returned to the main process, is ofconsiderable interest: The phosphate saved represents a substantialvalue, and it reduces a problem of deposition.

[0028] The procedure according with the present invention shall now beillustrated by means of the following and not-limiting example:

EXAMPLE

[0029] An experiment was performed in order to show the advantages ofthe present invention.

[0030] Solid particles are removed from the evaporated suspension in themain process by filtration. In the present case, the dried“regeneration-precipitate” composition was

[0031] 28.3 weight-% Na₂SO₄

[0032] 60.5 weight-% Na₂HPO₄

[0033] 7.5 weight-% Na₂S₂O₅

[0034] All phosphate is here assumed to be Na₂HPO₄.

[0035] The procedure was the following:

[0036] 671 gram of precipitate was separated from the SO₂ loaded bufferafter evaporation to a specific density of 1.90 kg/liter. After beingcrushed to a small particle size, the solid was transferred to a vesselprovided with a heating mantle and a magnetic stirrer. Water kept at 70°C. from a thermostat was circulated through the heating mantle. Then 360gram of water was added. After 30 minutes of stirring, the addition offinely divided SO₂ was sparged into the suspension. The SO₂ flow wasstopped after 90 minutes. Altogether 161 grams of SO₂ was introduced.This amounts to a somewhat lesser amount than required if all phosphatein the starting salt was in the form of Na₂HPO₄. It is assumed that someNaH₂PO₄ was present at start. After the SO₂ treatment, the remainingsolid material was separated from its mother liquid by filtration atelevated pressure with nitrogen as driving gas. The pressure filter wasof stainless steel, and was placed in a water bath kept at 70° C. Thefilter cake was washed on the filter with 100 milliliters of water at70° C. The cake was then analyzed for phosphate and sulphate. Thesulphate analysis followed the standard gravimetric procedure afterprecipitation as barium sulphate, while the determination of phosphatefollowed a standard calorimetric procedure after dissolving an aliquotein a “P-reagent” solution.

[0037] The final pressure-filtered material had the followingcomposition:

[0038] 79.1 weight-% Na₂SO₄

[0039] 18.0 weight-% Na₂HPO₄

[0040] 5.1 weight-% Na₂S₂O₅

[0041] Sum 102.2%, which is acceptable. 97.2% of the Na₂SO₄ at start wasleft in the final filter cake, while 89.7% of Na₂HPO₄ was found in thefiltrate.

1. A-procedure for removal of sodium sulphate from a phosphate solutionwhich has been used for absorption of SO₂ from flue gases, whereby aprecipitate forms when the absorption solution is regenerated byevaporation, wherein a part of the precipitate is separated from itssolution and treated with water and SO₂ for transformation ofdi-sodium-hydrogen-phosphate into mono-sodium-di-hydrogen-phosphate inthe aqueous solution, whereby the sodium sulphate remains undissolvedand is removed by filtration, whereafter the filtrate is added back tothe main process.
 2. The procedure of claim 1, wherein the filter cakeis treated with water for dissolving and recovering the rest amounts ofsodium pyrosulphite/sodium hydrogen sulphite
 3. The procedure of thepreceeding claims, wherein the treatment is accomplished at an elevatedtemperature.
 4. The procedure of claim 3, wherein the procedure iscarried out at a temperature below 100° C.
 5. The procedure of claim 4,wherein the procedure is carried out at temperatures between 40 and 80°C.